This invention relates to an apparatus for generating a laser beam having a high transmission efficiency.
A wave guide tube for transmitting a laser beam is known. The laser beam passes through the wave guide tube while being reflected by an inner peripheral surface of the wave guide tube. It is advantageous that the wave guide tube has a circular shape in section. The reasons are that a sectionally circular wave guide tube can be bent freely in any direction and that bending resistances of the wave guide tube are constant in all directions.
In order to enhance the transmission efficiency at the time when the laser beam passes through the sectionally circular wave guide tube, a tangentially polarized laser beam (laser beam of TE.sub.01 mode) is most preferable. Applied Physics Letters Vol. 38(10) 1981 carries an article titled "Low-order TE.sub.0Q operation of a CO.sub.2 laser for transmission through circular metallic waveguides", In this article, there is disclosed a CO.sub.2 laser beam generator made for the purpose of generating a laser beam of TE.sub.0Q mode. This laser beam generator comprises a discharge tube containing a CO.sub.2 gas. A first mirror and a convex lens are disposed at opposite ends of this discharge tube. A straight metal tube and a second mirror are disposed axially of this discharge tube. Reflecting surfaces of the first and second mirrors are held opposite to each other. The above convex lens and metal tube are disposed between the first and second mirrors. A laser beam reflected by the first mirror in parallel relation to an optical axis of this laser beam is made incident to and condensed by the convex lens, and then supplied to one end portion of the metal tube. The laser beam thus supplied to the metal tube passes therethrough while being reflected by an inner peripheral surface of this metal tube, and is then emitted from the other end of the metal tube. This laser beam is reflected by the second mirror so as to enter again into the other end of the metal tube. The laser beam thus entered into the metal tube passes therethrough while being reflected by the inner peripheral surface of this metal tube as in the manner above mentioned, and then proceeds from the above mentioned one end toward the convex lens while being diverged. This laser beam is made parallel to the above optical axis by the convex lens and caused to proceed toward the first mirror so as to be reflected by this first mirror. During such reciprocating process of the laser beam between the first and second mirrors, the laser beam is amplified, and polarized into TE.sub.0Q mode by the metal tube. One of the first and second mirrors is of a semitransparent type. The above amplified laser beam is allowed to transmit through this semitransparent mirror and outputted.
Although the first mirror has a concave reflecting surface, this is only for the purpose of correcting the divergence of the laser beam so as to hold it in parallel relation to the optical axis, and the concave reflecting surface does not have the function for tangentially polarizing the laser beam. Since the radius of curvature of the reflecting surface of the first mirror is approximately 5 m whereas the diameter of the discharge tube is approximately 11 mm, this reflecting surface can be regarded as a generally planar surface.
In the above laser beam generator, since the metal tube for making a laser beam into TE.sub.0Q mode is required to be disposed linearly of and away from the discharge tube, the overall length of the laser beam generator is obliged to become long. Moreover, when the generated laser beam is transmitted through the sectionally circular wave guide tube, the transmission efficiency is high if the wave guide tube is straight, but the transmission efficiency is significantly lowered if the wave guide tube is bent at a small radius of curvature.